I agree with Brumbull. It's good time to dig up your lawn....I would trench in, upside down, your existing turf, and remove large rubble. How big is your lawn? If possible I would mix in mushroom compost or similar too. Spend time getting it level by regular raking to get good tilth. And a general fertiliser a week,or so before seed sowing. If its not too big, lawn preparation is a very satisfying project. Can you keep new lawn watered reasonably well?